Production of alkyl nitrates



PRODUCTION OF ALKYL NITRATES Charles P. Spaeth, Woodbury, N. J., assignor to E. I. du Pont de Nemours and Company, Wilmington, DeL, a corporation of Delaware No Drawing. Application May 27, 1953, Serial No. 357,879

8 Claims. (Cl. 260-466) nited States PfllfifltO variety of other purposes, and are relatively simple to prepare on a laboratory scale by routine experimental procedures. Unfortunately, however, most of the laboratory methods of preparing such nitrates cannot be used on a commercial scale because of factors such as the instability of the waste acid, fire hazard, violent oxidative decomposition, drowning requirements which cause recovery of the unused acids to be uneconomical and/ or present waste disposal problems.

There have been a number of proposals made to overcome the foregoing factors, with varying degrees of success. The preparation of the nitrates of aliphatic alcohols has not been successful because of the tendency for oxidizing side reactions to consume the starting material. Thus, it has been proposed to operate with mixed acids at low temperatures, e. g., 10 C., to reduce side reaction, or, in the alternative, to operate with high mole ratio of HNOa and without the presence of sulfuric acid by removing the alkyl nitrate from the esterification vessel promptly in the form of an azeotrope with water. Obviously, low temperature operations require careful temperature control of the reaction, and are more difficult to carry out commercially than operations at moderate temperatures. The use of high mole ratios of nitric acid, on the other hand, increases the hazard because of the presence of the great excess of oxidizing ingredient in the reaction zone in the event the reaction should get out of control.

The use of urea to prevent the formation of nitrous acid and alkyl nitrites is well known. However, when urea is added to nitric acid, urea nitrate is formed, and in the nitric acid reaction medium, such formation decreases the available nitric acid for combination with the alkyl group.

It is, accordingly, an object of the present invention to provide an improved process for the preparation of alkyl nitrates. A further object of the present invention is to provide an eflicient mixed-acid process for preparing alkyl nitrates wherein the reaction is performed at temperatures practical on large industrial scale operation. Additional objects will become apparent as the invention is further described.

I have found that the foregoing objects may be achieved when I introduce an alcohol having the desired alkyl group into a reaction medium containing by weight of from 15 to 50% sulfuric acid, from 10 to 30% nitric acid, from 30 to 50% water, and at least 1% urea, the reaction medium being maintained at a temperature between 65 and 110 C. until the alkyl nitrate formed is separated therefrom. The alkyl nitrate may be separated by distillation, by settling and withdrawal of the alkyl nitrate layer, by centrifuging, or by solvent extraction.

2,768,964 Patented Oct. 30, 1956 For simplicity, I prefer to separate by distillation, using reduced pressure if necessary to remove the product at an economical rate without exceeding the upper temperature limit in the reaction zone.

In order to more fully describe this invention, reference is made to the following examples, which are illustrative of specific embodiments, the scope of the invention not being limited thereto. The percentages given are all by weight.

Example 1 The following table illustrates the effect of variations in acid ratios in the preparation of n-propyl nitrate. In each run, 60 parts by weight of n-propanol was slowly introduced below the surface level of the indicated reaction medium which Was contained in a reactor and maintained at a temperature between 85-95 C. and at a pressure of about 440 mm. of mercury. The propyl nitrate formed distilled off along with water and small quantities of unreacted n-propanol and nitric acid.

Reaction Zone Composition n-Propyl Nitrate Percent Percent Percent Percet t Percent Percent HNOa H2804 H O Urea Conversion Yield I From the foregoing table, the influence of the sulfuric acid on the conversion and yield is strikingly apparent. It will be noted that only after the quantity of sulfuric acid drops below 15% is'the drop in conversion very great. In those cases where conversions up to about are obtained, the separation of the nitrate ester is accomplished by simple stratification, and the n-propyl nitrate is obtained in a substantially anhydrous form.

Example 2 i Run #1 Run #2 Run #3 Run #4 Reaction Temperature 0.). 98-100 93-95 80-81 70-71 Distillation Pressure (mm). 471-472 440-443 299-300 149 Conversion (Percent) 88. 5 89. 9 88.1 78. 3 Yield (Percent) 97.3 96.1 94.2 99. 9

Example 3 Following the procedure set forth in Example 1, with a reaction medium of 28.7% nitric acid, 20% sulfuric acid, 44% Water and 7.3% urea, the reaction medium being maintained between 88-92 C., the following alcohols were reacted to the corresponding nitrates at the indicated conversion and yield:

sisting of 20.0% sulfuric acid, 28.7% nitric acid, 44%

water and 7.3% urea, and the medium maintained at a temperature of 9 l'93 C. 60.3 parts of. n-propanol, 57.6 parts of nitric: acid and- 24.9 parts of water were added concurrently over a period of 1%.- hours, the propyl nitrate formed being distilled along with water and unreacted alcohol and nitric acid at a pressure corresponding to 442 mm. of mercury. The conversion thus obtained was 90.4% with a yield. of 98.0%. This example illustrates a continuous operation utilizing the process of the present invention.

From the foregoing examples, it can be. seen that the present process is applicable to primary and secondary aliphatic alcohols and to cycloaliphatic alcohol. The conditions used are not favorable for the nitration of tertiary alcoholsbecause the reaction apparently preferentially forms an unsaturated compound which in effect destroys the alcohol before it can bev nitrated. The.

present process is most effective with. alcohols having no more than 8' carbon atoms, since the alcoholshaving more carbon atoms are much more dilficult to nitrate due to their insolubility in the nitrating mixture.

The foregoing examples also illustrate the fact that the present process can be operated either batch-wise or continuously. For'jbatch-wi'se operation it is apparent that a larger quantity of the reaction medium is required, since the nitric acid consumed during esterification isv not being replaced. For continuous operation, nitric acid and water are introduced concurrently with the alcohol at such rate as to maintain essentiallythe same reaction medium ratios with respect to nitric acid and water throughout the esterification. The acid and water will normally be introduced-in the form of aqueous nitric acid, although some of the water may be introduced with the alcohol. Obviously, the present process can also be operated by intermittent introduction of nitric acid and water to replace that removed by the esterification and distillation. Urea may be introduced any time or in any manner as required.

The presence. of a stabilizer such as urea is essential to the present process, and the temperature limits are based on the activity of urea as a stabilizer. It has been known for many years that urea could be used in the nitration of alcohols with nitric acid alone, and that its presence inhibited the undesirable oxidative side reaction. However, the urea tied up a mole equivalent quantity of nitric acid, thus requiring the use of more nitric acid. Urea was not effective in the mixed acid process as practiced in the prior art, i. e., at temperatures below 25 C. I have found that, surprisingly, urea will act effectively as a stabilizer for the nitration reaction only at a temperature above about 50 60 C;, and that its effectiveness rises as the temperature is increased. At temperatures above 110 C., the rate of hydrolysis of urea in acid solutions increases rapidly causing undue losses of urea, so that continuous addition would be required to maintain a sufiicient quantity for effective stabilization Within the reaction zone. The minimum quantity of urea required safety-wise for stabilization throughout the esterification is at least 1% of the total reaction mixture. In the present process, due to the presence of the sulfuric acid, the nitration; effectiveness of the nitric acid is not decreased by the presence of an excess of urea. Therefore, I prefer to insure against the depletion of the stabilizer by originally introducing an excess of urea, e. g., from 5 to 10% by weight of the. reaction medium, or by introducing additional urea during the esterification.

The invention. has been adequately described in the foregoing and illustrated by specific examples. I intend, therefore, to be limitedonly by the following claims.

I claim:

1. A process for the preparation of alkyl nitrates which comprises introducing at least one monohydric alcohol selected from the group consisting of primary alkyl alcohols, secondary alkyl alcohols, and cycloalkyl alcohols, wherein said alcohol contains less than nine carbon 1 atoms, into areaction medium comprising by weight of from 15 to 50% sulfuric acid, 10 to 30% nitric acid, 30 to 50% water and from 1 to 10% urea, said medium being maintained at a temperature between and 2. A process as claimed in claim 1, wherein the alkyl nitrate formed is separated from the reaction medium by distillation. z

3. A process as claimed in claim 1, wherein the alkyl nitrate formed is separated from the reaction medium by distillation, and nitric acid and water are introduced concurrently with the said alcohol in such amount as to maintain the nitric acid-Water .ratio of the reaction medium substantially constant.

4. A process as claimed in claim 1, wherein the alcohol is n-propanol. V

5. A process as claimed in claim 1, wherein the alcohol is isopropanol.

6. A process as claimed in claim 1 wherein the alcohol is cyclohexanol.

7. A process as claimed in claim 1', wherein the alcohols are a mixture of pentanols;

8. A process as claimed in claim I, wherein the alcohol is ethanol.

References Cited in the file of this patent Allan et al. Aug. 4, 1953 

1. A PROCESS FOR THE PREPARATION OF ALKYL NITRATES WHICH COMPRISES INTRODUCING AT LEAST ONE MONOHYDRIC ALCOHOL SELECTED FROM THE GROUP CONSISTING OF PRIMARY ALKYL ALCOHOLS, SECONDARY ALKYL ALCOHOLS, AND CYCLOALKYL ALCOHOLS, WHEREIN SAID ALCOHOL CONTAINS LESS THAN NINE CARBON ATOMS, INTO A REACTION MEDIUM COMPRISING BY WEIGHT OF FROM 15 TO 50% SULFURIC ACID, 10 TO 30% NITRIC ACID, 30 TO 50% WATER AND FROM 1 TO 10% UREA, AND MEDIUM BEING MAINTAINED AT A TEMPERATURE BETWEEN 65* AND 110* C. 